1. Field of the Invention
The present invention relates to cutting tools and in particular to cutting tools known as “table saws” having a table and a circular rotary cutting blade, so that a workpiece is cut by the cutting blade as it is moved along the table.
2. Description of the Related Art
Conventionally, as shown in FIGS. 51 and 52, a table saw 100 includes a table 102 for placing a workpiece W thereon, and a circular cutting blade 103 rotatably mounted to the table 102. The cutting blade 103 is positioned such that an upper portion of the cutting blade 103 extends above an upper surface of the table 102. The vertical position of the cutter blade 103 relative to the table 102 can be adjusted in order to adjust the height of the cutter blade 103 relative to the upper surface of the table 102. Pressing the workpiece W against the exposed upper portion of the cutter blade 103 can cut a workpiece W placed on the table 102. In this type of table saw 100, a cutting blade having a thickness of about 2 mm is typically used as a cutting blade 103.
Additionally in this kind of table saw 100, a flat, plate-like riving knife 104 is disposed on the rear side (i.e., the right side as viewed in FIGS. 51 and 52) of the cutting blade 103. The riving knife 104 is positioned within the same plane as the cutting blade 103. In general, the riving knife 104 is made of thin steel plate and has a thickness of about 2 mm corresponding to the cutting blade 103. As the workpiece W is moved during a cutting operation by the cutting blade 103, the riving knife 104 may consequently enter the kerf or split formed in the workpiece W by the cutting blade 103. This result is due to the riving knife 104 being positioned within the same plane as the cutting blade 103. The riving knife 104 entering the split helps to prevent cut portions of the workpiece positioned on either side of the cutting blade 103 from contacting the side surfaces of the cutting blade 103. As a result, a phenomenon can be prevented commonly known as “kickback,” i.e., causing the workpiece W to be pressed back against the direction of feed by the rotating cutting blade 103.
Typically, in this kind of table saw 100, a safety cover C is provided in order to cover the upper portion of the cutting blade 103 extending above the upper surface of the table 102. For example, the cover C may be formed from a material such as a transparent resin plate. The cover C may serve as a safety cover for preventing foreign objects from unintentionally contacting the cutting blade 103. Additionally, the cover C may also serve as a dust cover for preventing cutting chips produced during the cutting operation from being scattered to the surrounding environment.
When the workpiece W is initially moved during a cutting operation, the cover C may be opened upward as it is lifted by the workpiece W. During the cutting operation, the cover C may then be held so as to rest against the upper surface of the cut workpiece W. As the workpiece W is moved away from a the cutting blade 103 after having been cut, the cover C may again contact the upper surface of the table 102 in order to substantially cover the entire upper portion of the circular saw blade 103 mat extends above the upper surface of the table 102.
In general, the cover C is pivotally supported by a support block 106, via a pin 106a that is disposed on the rear side of the support block 106, for the purpose of ensuring mounting strength. The result is that the cover C is mounted to the upper portion of the riving knife 104 via the support block 106. In the known art, the support block 106 is fixedly mounted to the upper portion of the riving knife 104, for example, by means of two fixing screws 107. Therefore, as shown in FIGS. 53 and 54, nuts 108 may be secured to the upper portion of the riving knife 104 by welding, allowing the fixing screws 107 to engage the nuts 108 so as to be tightened. Taking into account the possible vibrations present during a cutting operation, there has been a proposal to securely fix the support block and the riving knife together through the use of rivets or the like, without utilizing bolts and nuts.
In the case of an ordinary cutting or ripping operation, the above known mounting structures of the cover C may not cause any problems in operation. However, in case of a groove forming or dado operation (i.e., any cut in which the height of the cutting blade 103 above the upper surface of the table 102 is less than the thickness of the workpiece W), the cover C and the support block 106 may be required to be removed due to their otherwise interfering with the workpiece W. For example, a riving knife 104 having nuts 108 as shown may interfere with a groove forming or dado operation since the nuts 108 extend laterally beyond width of the riving knife 104, and consequently, potentially beyond the width of the kerf or split.
Therefore, in the known art for a groove forming operation, not only are the cover C and the support block 106 removed from the table saw, but also the riving knife 104 itself is removed from the table saw. Alternatively, the removal of a single integrated assembly of a cover, a support block and a riving knife, has also been proposed. Such known techniques are disclosed in Japanese Laid-Open Utility Model Publication No. 6-46901 and U.S. Pat. No. 5,979,523.
As described above, the known table saws require troublesome and time-consuming operations for removing and remounting a riving knife in addition to a cover C and support block in order to perform a groove forming operation. Therefore, the overall ease of operability of the table saws is degraded.
In addition, the known table saws also require troublesome and time consuming operations for mounting a riving knife on the table saw. FIG. 55 shows a known table saw 300 having a table 302, a cutting blade 303, and a riving knife 304, corresponding to a table 102, a cutting blade 103, and a riving knife 104 as shown in FIG. 51. In this known table saw 300, a mount 305 is disposed below the table 302. The mount 305 is vertically movable relative to the table 302 along parallel support rods 306. The cutting blade 303 and an electric motor 307 for rotating the cutting blade 303 are mounted to the central portion of the mount 305. The mount 305 has an extension 305a on the left side as viewed in FIG. 55. The riving knife 304 is mounted to the extension 305a via a mounting bracket 309.
In general, the riving knife 304 is fixed to the mounting bracket 309 by fixing bolts 308. The riving knife 304 can therefore be fixed in position relative to the mounting bracket 309 by tightening the fixing bolts 308. Conversely, the riving knife 304 can be removed from the mounting bracket 309 by loosening the fixing bolts 308. In addition, the vertical position of the riving knife 304 can be adjusted within a predetermined range. Such a known mounting structure is disclosed in U.S. Pat. No. 6,170,370.
However, in order to mount and remove the riving knife 304, a spanner, wrench, Allen key, or the like, type of hand tool is required for tightening and loosening the fixing bolts 308. Therefore, the mounting and removing operations of the riving knife 304 can be troublesome and time-consuming. The operability of the table saw is also degraded in this respect.
Further, as described in connection with the known table saws shown in FIGS. 51 to 54, the nuts 108 are typically secured to the upper portion of the riving knife 104 by welding. The known mounting construction of the cover does not appear to be designed with the consideration that the cover will be removed from the riving knife. Instead, the cover is usually removed from the table saw together with the riving knife in the event of a groove forming operation.
In order to facilitate a groove forming operation, there has been proposed a riving knife divided into a first portion and a second portion that are positioned within a single plane. The cover of the cutting blade is vertically and pivotally supported on the second portion (e.g., located to the rear side of the first portion, or farthest from the cutting blade with respect to the cutting direction) of the riving knife. During the normal cutting operation, the first portion and the second portion cooperate together to operate as a single riving knife. In order to remove the cover for performing a groove forming operation, the cover may be removed together with the second portion of the riving knife. This allows the first portion of the riving knife, disposed on the front side of the second portion with respect to the cutting direction, to be used for a groove forming operation without having to be removed.
However, the divided riving knife causes an additional problem. Generally, in order to perform a riving function the riving knife is made of thin steel plate having a thickness equal to or smaller than the thickness of a cutting blade (i.e., 1.5 mm to 2 mm in general). As a result of such a thin thickness, there is a difficulty in maintaining the positions of the first and second portions of the riving knife within a single plane. In some cases, the separate components of the riving knife may not smoothly enter the split of the workpiece. The operability of the table saw is subsequently degraded in this respect and a cutting operation cannot be efficiently performed.